One such CVT, well known to those skilled in the art, is represented in highly schematic form in FIG. 1. An engine 10 drives a transmission input shaft 12. A transmission output shaft 14 is coupled to wheels 16 of a motor vehicle. Stepless variation of transmission ratio is provided by means of a variator 18. The word “variator” will be used herein to refer to a device which transfers drive between a first rotary member (such as variator input shaft 20) and a second rotary member (such as output shaft 22) at a steplessly variable speed ratio. The transmission also has an epicyclic shunt gear train 24 (whose construction is not shown in this drawing) having three shafts. The shunt's first shaft 26 is operatively coupled to the engine 10, e.g. through fixed ratio gearing. The shunt's shaft 28 is coupled to the output shaft 22 of the variator 18. The third shaft 30 rotates at a speed which is a function of the speeds of the first and second shafts, and is coupled to the transmission output shaft 14.
A transmission of this type can provide an infinite speed reduction, referred to in the art as “geared neutral”. At some variator ratio, the speeds of the first and second rotary shafts 26, 28 of the shunt 24 cancel each other out, leaving the third shaft 30—and the transmission output—stationary, despite the fact that the output is not physically de-coupled from the moving engine. Typically gear ratios in the transmission are chosen such that merely by changing the variator a to the speed ratio provided by the transmission as a whole can be varied through a range of reverse and forward gears including geared neutral.
Power is recirculated through the variator 18 by the shunt 24, in a direction against the flow of power from engine to wheels (or vice versa, during engine braking). This power recirculation reduces the total power flow through the variator, which is thus required to transmit only part of the total power transmitted by the transmission. The variator is typically the least energy efficient part of the transmission. Hence it is desirable to minimise power flow through it.
Now, in some motor vehicles having a conventional stepped ratio main gearbox, a secondary gearbox is provided between the main gearbox and the wheels. Tractors often have this type of arrangement. For low speed operations such as ploughing, the secondary gearbox is placed in a low ratio. When higher speeds are needed, e.g. when driving on a road, the secondary gearbox is placed in a high ratio. A secondary gearbox can likewise be used with a CVT. In FIG. 1 such a gearbox is indicated in phantom at 32, and is switchable between low and high ranges.
Optimising efficiency of such a CVT presents a problem. Suppose that the low range is intended to provide vehicle speeds from 15 kph reverse to 15 kph forwards, and that the high range is intended to provide a forward speed up to 40 kph. Clearly this can be achieved by choice of suitable ratios for the secondary gearbox 32, in the type of arrangement seen in FIG. 1. In high range, the transmission would then be capable of providing vehicle speeds from 40 kph forwards to 40 kph in reverse. Such a high reverse speed is not however required for most applications, and transmission efficiency with such a system would be less than optimal. The proportion of total power handled by the variator increases with increasing transmission ratio range. Hence in the FIG. 1 transmission, and in high range, an unnecessarily large proportion of power is handled by the variator, impairing transmission efficiency.